Massive Dirac Fermion Behavior in a Low Bandgap Graphene Nanoribbon Near a Topological Phase Boundary

Graphene nanoribbons (GNRs) have attracted much interest due to their largely modifiable electronic properties. Manifestation of these properties requires atomically precise GNRs which can be achieved through a bottom–up synthesis approach. This has recently been applied to the synthesis of width-modulated GNRs hosting topological electronic quantum phases, with valence electronic properties that are well captured by the Su–Schrieffer–Heeger (SSH) model describing a 1D chain of interacting dimers. Here, ultralow bandgap GNRs with charge carriers behaving as massive Dirac fermions can be realized when their valence electrons represent an SSH chain close to the topological phase boundary, i.e., when the intra- and interdimer coupling become approximately equal. Such a system has been achieved via on-surface synthesis based on readily available pyrene-based precursors and the resulting GNRs are characterized by scanning probe methods. The pyrene-based GNRs (pGNRs) can be processed under ambient conditions and incorporated as the active material in a field effect transistor. A quasi-metallic transport behavior is observed at room temperature, whereas at low temperature, the pGNRs behave as quantum dots showing single-electron tunneling and Coulomb blockade. This study may enable the realization of devices based on carbon nanomaterials with exotic quantum properties.     

Low-temperature electronic transport in single K(0.27)MnO(2)·0.5H(2)O nanowires: enhanced electron-electron interaction

The current-voltage (I-V) characteristics and electrical resistivity of isolated potassium manganese oxide (K(0.27)MnO(2)·0.5H(2)O) nanowires prepared by a simple hydrothermal method were investigated over a wide temperature range from 300 to 4?K. With lowering temperature, a transition from linear to nonlinear I-V curves was observed around 50?K, and a clear zero bias anomaly (i.e., Coulomb gap-like structure) appeared on the differential conductance (dI/dV) curves, possibly due to enhanced electron-electron interaction at low temperatures. The temperature dependence of resistivity, [Formula: see text], follows the Efros-Shklovskii (ES) law, as expected in the presence of a Coulomb gap. Here we note that both the ES law and Coulomb blockade can in principle lead to a reduced zero bias conductance at low temperatures; in this study we cannot exclude the possibility of Coulomb-blockade transport in the measured nanowires, especially in the low-temperature range. It is still an open question how to pin down the origin of the observed reduction to a Coulomb gap (ES law) or Coulomb blockade.     

Dynamic Transport of Interacting Electrons in a Mesoscopic Quantum Wire

Electron density distribution in a quantum wire (QW) coupledto reservoirs is investigated, treating this structure as aunified quantum system and taking into account the Coulombinteraction of electrons. We show that electrons aretransferred between the QW and the reservoirs when theequilibrium state is established. As a result the QWacquires a positive or negative charge or remains neutral as awhole, depending on the QW size and the background charge.Electron transport in a QW is investigated for the lattercase using the model which allows one to treat exactly theCoulomb interaction. We study the Coulomb interaction effecton the dynamic conductance. The real part of the impedanceshows a resonant behavior versus the frequency which is causedby the reflection of the charge waves from the contacts andtheir interference in the QW. The Coulomb interaction effectconsists in a nonlinear dependence of the resonant frequencieson the wave number. No effective interaction parameter of theLuttinger liquid can simulate the frequency dependence of theimpedance calculated with the real Coulomb interaction.     

Ambipolar Coulomb blockade characteristics in a two-dimensional Si multidot device

A two-dimensional Si multidot channel field-effect transistor is fabricated from a silicon-on-insulator material and the electrical characteristics are studied. The multidots are formed using a nanometer-scale local oxidation of Si process developed in our laboratory. The device shows ambipolar characteristics because of Schottky source and drain, i.e., the carriers are electrons for positive gate voltage and holes for the negative one. It is shown that Coulomb blockade (CB) oscillations are clearly observed for both of the electrons and holes at measurement temperatures up to 60 K. Both CB characteristics show nonperiodic oscillation and an open Coulomb diamond. These features are ascribed to the single electron/hole tunneling in the Si multidot channel.     

Privacy-Preserving Decision Protocols Based on Quantum Oblivious Key Distribution

Oblivious key transfer (OKT) is a fundamental problem in the field of secure multi-party computation. It makes the provider send a secret key sequence to the user obliviously, i.e., the user may only get almost one bit key in the sequence which is unknown to the provider. Recently, a number of works have sought to establish the corresponding quantum oblivious key transfer model and rename it as quantum oblivious key distribution (QOKD) from the well-known expression of quantum key distribution (QKD). In this paper, a new QOKD model is firstly proposed for the provider and user with limited quantum capabilities, where both of them just perform computational basis measurement for single photons. Then we show that the privacy for both of them can be protected, since the probability of getting other’s raw-key bits without being detected is exponentially small. Furthermore, we give the solutions to some special decision problems such as set-member decision and point-inclusion by announcing the improved shifting strategies followed QOKD. Finally, the further discussions and applications of our ideas have been presented.     

Double-sided transparent electrodes of TiO2 nanotube arrays for highly efficient CdS quantum dot-sensitized photoelectrodes

A novel double-sided CdS quantum dots-sensitized TiO₂ nanotube (TNT)/ITO photoelectrode is fabricated to improve the energy conversion efficiencies of quantum dots-sensitized solar cells (QDSCs). Our experimental results show that the double-sided CdS quantum dots-sensitized TNT/ITO photoelectrodes show enhanced light absorption. As a consequence, the photoelectrochemical response of the CdS/TNT/ITO photoelectrode is much improved compared with single-sided CdS sensitized TNT arrays on Ti substrate (i.e., CdS/TNT/Ti photoelectrode). An optimum conversion efficiency of 7.5 % is achieved by the double-sided CdS(15)/TNT/ITO photoelectrode, which is an enhancement of about 120 % when compared with the single-sided CdS/TNT/Ti photoelectrode. Our results demonstrate that the energy conversion efficiencies of QDSCs can be improved by designing a new photoelectrode structure.     

Motion of a “drift island” in a toroidal magnetic confinement device with Coulomb scattering of the particles

A resonant drift trajectory of a charged-particle in a magnetic field (a “drift island”) can be used to remove high-energy impurities from a thermonuclear plasma and to introduce (inject) high-energy particles into the plasma. As a rule, these effects are studied neglecting the Coulomb scattering, i.e., in the collisionless approximation. In the present letter, the effect of Coulomb scattering of a particle with a resonant trajectory by plasma particles is studied. The conditions under which the drift resonance is maintained are found, i.e., the plasma densities and plasma density profiles for which the “drift island” can still move over the transverse section of the plasma are determined. Pis’ma Zh. Tekh. Fiz. 25, 19–27 (September 26, 1999)     

Fabrication and Characterization of Sidewall Defined Silicon-on-Insulator Single-Electron Transistor

We reported the fabrication and characterization of a new type of silicon-on-insulator (SOI) single-electron transistor utilizing usual CMOS sidewall gate structures. We used oxide sidewall spacer layers as well as two poly-Si finger gates on an SOI wire mesa as implantation masks, to form tunnel barriers and thus a quantum dot (QD) that is smaller than the spacing between polygates. Characterization results exhibited clear Coulomb oscillations persisting up to 30 K. The Coulomb energy and the size of the QD extracted from three devices were consistent with the spacing between two poly-Si gates of each device. Furthermore, the junction capacitance of each device was almost constant and only the gate capacitance varied. These analyses suggested that the size of the QD was fully controlled by the process.      

Analysis of Energy Quantization Effects on Single-Electron Transistor Circuits

In this paper, the effects of energy quantization on different single-electron transistor (SET) circuits (logic inverter, current-biased circuits, and hybrid MOS-SET circuits) are analyzed through analytical modeling and Monte Carlo simulations. It is shown that energy quantization mainly increases the Coulomb blockade area and Coulomb blockade oscillation periodicity, and thus, affects the SET circuit performance. A new model for the noise margin of the SET inverter is proposed, which includes the energy quantization effects. Using the noise margin as a metric, the robustness of the SET inverter is studied against the effects of energy quantization. An analytical expression is developed, which explicitly defines the maximum energy quantization (termed as “ quantization threshold”) that an SET inverter can withstand before its noise margin falls below a specified tolerance level. The effects of energy quantization are further studied for the current-biased negative differential resistance (NDR) circuit and hybrid SETMOS circuit. A new model for the conductance of NDR characteristics is also formulated that explains the energy quantization effects.      

Digital switching in the quantum domain

Presents a switching architecture such that digital data can be switched in the quantum domain. The proposed mechanism supports unicasting as well as multicasting, and is strict-sense nonblocking. In addition, with appropriate interface conversion, this architecture can also be used to switch classical information. This results in a quantum switch that can be used to build classical and quantum information networks. To present this idea, we define the connection digraph which can be used to describe the behavior of a switch at a given time, then we show how a connection digraph can be implemented using elementary quantum gates. Compared with a traditional space or time domain switch, the proposed switching mechanism is much more scalable. Assuming an n/spl times/n quantum switch, the space consumption grows linearly, i.e., O(n), while the time complexity is O(1) for unicasting, and O(log/sub 2/n) for multicasting. Based on these advantages, a high-throughput switching device can be built simply by increasing the number of I/O ports.     

The role of Coulomb interaction in thermoelectric effects of an Aharonov-Bohm interferometer

We investigate the thermoelectric effects of an Aharonov-Bohm (AB) interferometer with a quantum dot (QD) embedded in each of its arms, where the intra-dot Coulomb interaction between electrons in each QD is taken into account. Using Green's function methods and the equation of motion (EOM) technique, we find that the Seebeck coefficient and Lorenz number can be strongly enhanced when the chemical potential sweeps the molecular states associated with the Fano line-shapes in the transmission spectra, due to quantum interference effects between the bonding and antibonding molecular states. It is found that enhancement of the thermoelectric effects occurs between the two groups of conductance peaks in the presence of strong intra-dot Coulomb interaction-the reason being that a transmission node is developed in the Coulomb blockade regime. In this case, the maximum value of the Lorenz number approaches 10π(2)k(B)(2)/(3e(2)). Its thermoelectric conversion efficiency in the absence of phonon thermal conductance, described by the figure of merit ZT, approaches 2 at room temperature. Therefore, it may be used as a high-efficiency solid-state thermoelectric conversion device under certain circumstances.     

Superconducting and Insulating Behavior in One-Dimensional Josephson Junction Arrays

Experiments on one-dimensional small capacitance JosephsonJunction arrays are described. The arrays have a junctioncapacitance that is much larger than the stray capacitance ofthe electrodes, which we argue is important for observation ofCoulomb blockade. The Josephson energy can be tuned in situand an evolution from Josephson-like to Coulomb blockadebehavior is observed. This evolution can be described as asuperconducting to insulating, quantum phase transition. Inthe Coulomb blockade state, hysteretic current-voltagecharacteristics are described by a dynamic model which is dualto the resistively shunted junction model of classicalJosephson junctions.     

Tunable graphene single electron transistor

We report electronic transport experiments on a graphene single electron transistor. The device consists of a graphene island connected to source and drain electrodes via two narrow graphene constrictions. It is electrostatically tunable by three lateral graphene gates and an additional back gate. The tunneling coupling is a strongly nonmonotonic function of gate voltage indicating the presence of localized states in the barriers. We investigate energy scales for the tunneling gap, the resonances in the constrictions, and for the Coulomb blockade resonances. From Coulomb diamond measurements in different device configurations (i.e., barrier configurations) we extract a charging energy of approximately 3.4 meV and estimate a characteristic energy scale for the constriction resonances of approximately 10 meV.     

Advanced modeling techniques for micromagnetic systems

We present a review of micromagnetic and magnetotransport modeling methods which go beyond the standard model. We first give a brief overview of the standard micromagnetic model, which for (i) the steady-state (equilibrium) solution is based on the minimization of the free energy functional, and for (ii) the dynamical solution, relies on the numerical solution of the Landau-Lifshitz-Gilbert (LLG) equation. We present three complements to the standard model, i.e., (i) magnetotransport calculations based on ohmic conduction in the presence of the anisotropic magnetoresistance (AMR) effect, (ii) magnetotransport calculations based on spin-dependent tunneling in the presence of single charge tunneling (Coulomb blockade) effect, and (iii) stochastic micromagnetics, which incorporates the effects of thermal fluctuations via a white-noise thermal field in the LLG equation. All three complements are of practical importance: (i) magnetotransport model either in the ohmic or tunneling transport regimes, enables the conversion of the micromagnetic results to the measurable quantity of magnetoresistance ratio, while (ii) stochastic modeling is essential as the dimensions of the micromagnetic system reduces to the deep submicron regime and approaches the superparamagnetic limit.     

Charging and Spin Effects in Triple Dot Artificial Molecules

We have fabricated artificial molecules consisting of three coupled quantum dots defined in the two-dimensional electron gas of a GaAs/AlGaAs heterostructure using lithographically patterned gates and trenches. The three dots are arranged in a ring structure, where each dot is coupled to the other two dots. We find that, when tuned to the Coulomb blockade regime, the triple quantum dot device acts as a charge rectifier: an electron enters the third dot where it is trapped, producing a jamming effect where no other electron may enter the first dot. Triple quantum dots coupled in a ring will allow for the study of new molecular phases using artificial molecules and may also serve as building blocks of two-dimensional arrays for quantum computation.     

Charging and Spin Effects in Triple Dot Artificial Molecules

We have fabricated artificial molecules consisting of three coupled quantum dots defined in the two-dimensional electron gas of a GaAs/AlGaAs heterostructure using lithographically patterned gates and trenches. The three dots are arranged in a ring structure, where each dot is coupled to the other two dots. We find that, when tuned to the Coulomb blockade regime, the triple quantum dot device acts as a charge rectifier: an electron enters the third dot where it is trapped, producing a jamming effect where no other electron may enter the first dot. Triple quantum dots coupled in a ring will allow for the study of new molecular phases using artificial molecules and may also serve as building blocks of two-dimensional arrays for quantum computation.     

Capacitance theory of open quantum systems with classical contacts

We consider semiconductor devices composed of a small quantum structure as the active device region and two classical environments constituting the source- and the drain contact. The contacts are taken as free electron gases with infinite conductivity defining the chemical potentials in the contacts. The transport through the quantum structure is described in the Landauer–Büttiker formalism using electronic scattering wave functions which determine the electron density in the quantum system. In our Hartree approximation these charges and the induced charges in the contacts are the sources of the self-consistent Coulomb field. As a particular quantum structure we study a GaAs heterostructure device consisting of a two-dimensional electron gas sandwiched between a gate contact and an AlGaAs blocking barrier [see V.T. Dolgopolov et al., Phys. Low-Dim. Struct. 6 (1996) 1]. We demonstrate the quantitative agreement of our theory with the experimental results.     

Room‐Temperature Mesoscopic Fluctuations and Coulomb Drag in Multilayer WSe2

Mesoscopic fluctuations, manifesting the quantum interference (QI) of electrons, have been theoretically proposed in bilayer Coulomb drag systems. Unfortunately, these phenomena are usually observed at cryogenic temperatures, which severely limits their novel physics for pragmatic applications. In this paper, observation of room‐temperature QI and Coulomb drag in a multilayer WSe₂ transistor is reported via graphene contacts separately at its top and bottom layers. The central layers of WSe₂ act as an insulating region with a width of few nanometers, which spatially separates the top and bottom conducting channels and provides a strong Coulomb interaction between them, leading to large conductance oscillations at room temperature. The gradual suppression of the oscillations with the increase in the applied magnetic field and/or injected current further confirms the QI phenomenon. With the decrease in temperature, the Coulomb drag effect is exhibited in the system owing to the increased thickness of the insulating region. This study reveals a novel approach for realization of advanced quantum electronics operating at high temperatures.     

Topographic curvature effects in applied avalanche modeling

This paper describes the implementation of topographic curvature effects within the RApid Mass MovementS (RAMMS) snow avalanche simulation toolbox. RAMMS is based on a model similar to shallow water equations with a Coulomb friction relation and the velocity dependent Voellmy drag. It is used for snow avalanche risk assessment in Switzerland. The snow avalanche simulation relies on back calculation of observed avalanches. The calibration of the friction parameters depends on characteristics of the avalanche track. The topographic curvature terms are not yet included in the above mentioned classical model. Here, we fundamentally improve this model by mathematically and physically including the topographic curvature effects. By decomposing the velocity dependent friction into a topography dependent term that accounts for a curvature enhancement in the Coulomb friction, and a topography independent contribution similar to the classical Voellmy drag, we construct a general curvature dependent frictional resistance, and thus propose new extended model equations. With three site-specific examples, we compare the apparent frictional resistance of the new approach, which includes topographic curvature effects, to the classical one. Our simulation results demonstrate substantial effects of the curvature on the flow dynamics e.g., the dynamic pressure distribution along the slope. The comparison of resistance coefficients between the two models demonstrates that the physically based extension presents an improvement to the classical approach. Furthermore a practical example highlights its influence on the pressure outline in the run out zone of the avalanche. Snow avalanche dynamics modeling natural terrain curvature centrifugal force friction coefficients.     

Scanning gate imaging of quantum dots in 1D ultra-thin InAs/InP nanowires

We use a scanning gate microscope (SGM) to characterize one-dimensional ultra-thin (diameter ≈ 30 nm) InAs/InP heterostructure nanowires containing a nominally 300 nm long InAs quantum dot defined by two InP tunnel barriers. Measurements of Coulomb blockade conductance versus backgate voltage with no tip present are difficult to decipher. Using the SGM tip as a charged movable gate, we are able to identify three quantum dots along the nanowire: the grown-in quantum dot and an additional quantum dot near each metal lead. The SGM conductance images are used to disentangle information about individual quantum dots and then to characterize each quantum dot using spatially resolved energy-level spectroscopy.